1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing thereof, and more particularly, to a semiconductor device having a resistor layer formed of polycrystalline silicon and a method of manufacturing thereof.
2. Description of the Background Art
A semiconductor device having a resistor element formed of polysilicon is conventionally known. FIG. 21 is a plan view of a semiconductor device having a conventional polysilicon resistor film. FIG. 22 is a sectional view of the semiconductor device of FIG. 21 taken along lines X--X. Referring to FIGS. 21 and 22, a conventional semiconductor device includes a silicon substrate 1, an insulation oxide film 2 formed on the silicon substrate 1, polysilicon resistor films 3a and 3b formed with a predetermined distance therebetween and extending in substantially the same direction on the insulation oxide film 2, an insulation oxide film 4 formed on the polysilicon resistor films 3a, 3b and on the insulation oxide film 2, a metal interconnection layer 5a formed on the insulation oxide film 4 to cover the polysilicon resistor film 3a, a metal interconnection layer 7a electrically connected to the polysilicon resistor film 3a, and a metal interconnection layer 7b electrically connected to the polysilicon resistor film 3b, and a plasma nitride film 6 formed to cover metal interconnection layers 5a, 7a and 7b.
The polysilicon resistor films 3a and 3b are used for the purpose such as defining the output value of a circuit by the resistance ratio thereof. FIG. 23 is a circuit diagram showing a constant voltage circuit using polysilicon resistor films 3a and 3b. Referring to FIG. 23, the polysilicon resistor films 3a and 3b are employed as resistances R.sub.A and R.sub.B, respectively. The output voltage V.sub.OUT is represented by the equation shown in FIG. 23 in which Q.sub.1, Q.sub.2 are transistors, R.sub.1, R.sub.A and R.sub.B are resistances, V.sub.CC is power supply voltage, and V.sub.BE is the base-emitter voltage of a transistor Q1. The output voltage V.sub.OUT is determined by the resistance ratio of resistance R.sub.A (polysilicon resistor film 3a) to resistance R.sub.B (polysilicon resistor film 3b). The resistance ratio is controlled by changing the length or width of polysilicon resistor film 3a and polysilicon resistor film 3b, respectively.
As described above, there are cases where the output value of a circuit is determined by the resistance ratio of a polysilicon resistor film 3a to a polysilicon resistor film 3b.
However, a semiconductor device having conventional polysilicon resistor films as shown in FIGS. 21 and 22 had problems as set forth in the following. FIG. 24 is a sectional view of a structure of a semiconductor device having conventional polysilicon resistor films for describing the problems. Referring to FIG. 24, a great amount of hydrogen ions H.sup.+ 10 is included in a plasma nitride film 6. More specifically, when plasma nitride film 6 is formed by a plasma CVD method, hydrogen components will be included in plasma nitride film 6. The details thereof is disclosed in J. Appl. Phys., Vol. 49, No. 4, April 1978 pp. 2473-2477. There was a disadvantage that these hydrogen ions H.sup.+ 10 are provided to the grain boundary of the polysilicon resistor films 3a and 3b. This is disclosed in J. Appl. Phys., Vol 63, No. 4, 15 February 1988 pp. 1117-1120, for example. When the formed plasma nitride film 6 was subjected to a temperature of not less than 380.degree. C. by a subsequent thermal process, there was a disadvantage of hydrogen ions H.sup.+ 10 in plasma nitride film 6 being supplied to polysilicon resistor films 3a and 3b. It is needless to say that plasma nitride film 6 is indispensable in LSI as a passivation film for preventing intrusion of external ions.
If hydrogen ions 10 are applied to the polysilicon resistor films 3a and 3b from the plasma nitride film 6, the resistance values of polysilicon resistor films 3a and 3b will change. Because a metal interconnection layer 5a is formed above the polysilicon layer 3a, the amount of hydrogen ion 10 provided from the plasma nitride film 6 to the polysilicon resistor film 3a is less than that provided to the polysilicon resistor film 3b. A difference in the amount of hydrogen ion 10 provided will result in difference in the amount of change of the resistance values between the polysilicon resistor film 3a and the polysilicon resistor film 3b. Therefore, the resistance ratio of the polysilicon resistor film 3a and the polysilicon resistor film 3b will be changed. In such a case, the output voltage V.sub.OUT in a constant voltage circuit such as that shown in FIG. 23 will be changed according to the change in the resistance ratio of resistance R.sub.A (polysilicon resistor film 3a) to resistance R.sub.B (polysilicon resistor film 3b). There was a problem that an output value conforming to the design can not be obtained if there is a change in the resistance ratio of the polysilicon resistor film 3a to the polysilicon resistor film 3b.
The resistance used in an LSI linear circuit is implemented with a pair of a plurality of lines to have the output voltage and current determined by the resistance ratio thereof. Therefore, if the resistance ratio is changed, a desired output could not be obtained.